The invention relates generally to detecting core faults and more particularly to testing laminated cores of electric machines for interlamination short circuits.
A conventional large generator lamination segment includes a back iron, teeth and slots. Lamination segments (each about 254 microns to about 508 microns thick) are formed into a magnetic core by stacking. Typically a plurality of lamination segments (eighteen lamination segments each being twenty degrees, for one example) are used to form a complete first lamination with the next plurality of lamination segments forming a complete second lamination on top of and offset from the lamination segments in the first lamination. The stacking continues until formation of a short stack of about 2.54 centimeters to about 10.16 centimeters thick. A plurality of short stacks are further joined and/or clamped by bolts and/or other mechanical devices to form a stator core. A typical large generator stator core has a diameter ranging from about 1 meter to about 3 meters and a length ranging from about 1 meter to about 10 meters.
Laminated stator cores are tested for interlamination shorts during manufacture and during maintenance or service procedures. Core faults, such as faults caused by short circuited laminations, in large electric machines can be highly destructive.
As described in commonly assigned Kliman et al., U.S. patent application Ser. No. 09/575,715, filed Jul. 28, 2000 and allowed Feb. 25, 2002, core faults may be detected by positioning a magnetic yoke wound by a winding near at least one tooth of the core; supplying current to the winding to inject magnetic flux into the at least one tooth; measuring at least one signal resulting from the injected magnetic flux; and using the measured signal to detect core faults. When a small portion of the core is excited, if the laminations are well insulated from each other, the flux response to the excitation will be primarily due to the permeable core material as modified by normal hysterisis losses and eddy currents in the laminations. However, if faults exist anywhere in the excited region, circulating currents will be induced which will alter the magnitude and phase of the response. Such altered phases or magnitudes can be used as a core condition indicator when used to compare one region of the core with another region of the core or to trend a single region over time. Additionally, analysis of the signal distribution for normal conditions and known fault conditions can be used to interpret measured signals in order to estimate core condition.
U.S. patent application Ser. No. 09/575,715 has several advantages as compared to prior embodiments. For example, short stacks of laminations can be tested individually while stacking during core fabrication and/or during core servicing so that, if a fault is located, remedial measures can be taken immediately on the affected lamination, and, if no fault is located, additional stacks can be formed and tested. In contrast, in prior embodiments, if a fault is later located in the middle of a completed core, a substantial portion of the core had to be un-stacked to gain access to the fault.
When the thickness of the magnetic yoke in U.S. patent application Ser. No. 09/575,715 exceeds the thickness of about two or three core laminations, sensitivity and selectivity are reduced since the magnetic yoke itself will influence the losses measured along the winding. Physical and practical constraints limit minimum yoke thicknesses however. Another factor that influences sensitivity is the distance between the magnetic yoke and the core laminations. About 50-75 micrometers of lamination stagger typically result from punching tolerances and assembly variability when fabricating laminated cores. In machines which function as generators, lamination core stagger is typically filled in and covered up by layers of thick paint. Such paint would further increase the effective gap between the magnetic yoke and the core.
It would be desirable to have a core fault detection method with increased sensitivity.
Briefly, in accordance with one embodiment of the present invention, a method for detecting core faults comprises (a) positioning a magnetic yoke near at least one tooth of the core, the magnetic yoke being wound by a winding and comprising two core-facing surfaces and at least one flux sensor situated on at least one of the two core-facing surfaces; (b) supplying current to the winding to inject magnetic flux into the at least one tooth of the core; (c) using the at least one flux sensor to measure a signal resulting from the injected magnetic flux; and (d) using the measured signal to detect variations in flux on the at least one core-facing surface representative of core faults.
In accordance with another embodiment of the present invention, a system for detecting core faults comprises at least one magnetic yoke for being positioned near at least one tooth of the core, the at least one magnetic yoke being wound by a winding and comprising two core-facing surfaces and at least one flux sensor situated on at least one of the core-facing surfaces.